Buck converters are an example for a DC-DC power converter. A buck converter may be operated in one or more modes, notably in a PFM (pulse frequency modulation) or in a PWM (pulse width modulation) mode. The PFM mode is typically used for relatively low load currents and the PWM mode is used for relatively high load currents.
FIG. 1 illustrates an example buck converter 100 which comprises a high side switch 106 (e.g. a PMOS, i.e. p-type metal oxide semiconductor, transistor) and a low side switch 107 (e.g. an NMOS, i.e. an n-type MOS, transistor). The switches 106, 107 are driven using respective gate drivers 104, 105. Furthermore, the buck converter 100 comprises an inductor 110 (e.g. a coil) which may be coupled to an integrated circuit (IC) comprising the switches 106, 107 via a pin 111. In addition, the buck converter 100 may comprise an output capacitor 112 which is arranged in parallel to an output of the buck converter 100.
In PFM mode, the buck converter 100 turns on the high side switch 106, when the output voltage 127 of the buck converter 100 falls below a reference voltage 122, which may be provided e.g. by a voltage DAC 101 (i.e. a voltage digital-to-analog-converter). In particular, the high side switch 106 may go into on-state, if it is detected by the comparator 102 that the feedback voltage 121 (which is derived from the output voltage 127 and/or which may be proportional or equal to the output voltage 127) falls below the reference voltage 122. This situation may be indicated by the under-voltage signal 123 at the output of the comparator 102.
Within the PFM mode, the high side switch 106 is turned off, when the current in the inductor 110 reaches a threshold value which may be referred to as a sleep current-limit or as a reference current 126. For this purpose, the buck converter 100 may comprise a current sense circuit 108 which is configured to monitor the current through the inductor 110. In particular, the current sense circuit 108 may be configured to provide a sensed current 128 which is indicative of (e.g. proportional or equal to) the current through the inductor 110. The sensed current 128 may be compared with a (fixed) reference current 126 (which is indicative of the sleep current-limit) to provide a current-limit signal 124, which is indicative of whether the sensed current 128 is greater than or smaller than the reference current 126.
As such, the high side switch 106 may be controlled using a control circuit 103 based on the current-limit signal 124 (for putting the high side switch 106 into off-state) and based on the under-voltage signal 123 (for putting the high side switch 106 into on-state).
The low side switch 107 may be turned on (i.e. may be put into on-state), when the high side switch 106 is turned off, in order to ensure that the high side switch 106 and the low side switch 107 are not in on-state concurrently. Furthermore, the low side switch 107 may be turned off, when the current in the inductor 110 is fully discharged. This condition may be detected based on the current-limit signal 124. In particular, in this case, the reference current 126 may be set to zero during the on-phase of the low side switch 107.
The PFM mode is typically not used for relatively large currents, and by consequence the current-limit, which is set by the reference current 126, is normally set relatively low to maximize efficiency of the buck converter 100 and to minimize the output ripples of the current/voltage at the output of the buck converter 100.
In PWM mode, the high side switch 106 is turned on based on a clock signal (which defines a certain pulse width). Furthermore, the high side switch 106 is turned off using a control loop (e.g. based on a pre-determined peak current through the inductor 110). In PWM mode, the low side switch 107 is typically controlled in a complementary manner with respect to the high side switch 106. In particular, the low side switch 107 is typically switched on, whenever the high side switch is off.
The PFM mode may be used for relatively low load currents and the PWM mode may be used for relatively high load currents. Problems may occur in case of load transients, e.g. when the load current rapidly increases from a relatively low value (when the power converter 100 is operated in PFM mode) to a relatively high value (when the power converter 100 should be operated in PWM mode). In such a situation, the output voltage 127 may decrease substantially (especially in the case if the load current increases to a value higher than the one given by the fixed reference current mentioned above) and the power converter 100 may not be able to perform a transition to PWM mode sufficiently fast, in order to provide the requested load current.